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Soil atmosphere concentration profiles and methane emission rates in the restoration covers above landfill sites: Equipment and preliminary results
Waste Management & Research (1990) 8, 21-31
SOIL ATMOSPHERE CONCENTRATION PROFILES AND METHANE EMISSION RATES IN THE RESTORATION COVERS ABOVE LANDFILL SITES : EQUIPMENT AND PRELIMINARY RESULTS Hilary A . Jones* and D. B. Nedwell** (Received March 1989, revised July 1989) In an investigation into the microbial oxidation of methane in soils, a simple and robust sampling device for the collection of small samples of soil atmosphere was used. The method has the potential for determining small-scale variations in depth profiles of gases, without the potential problems of sample migration during collection . Measurements of methane concentration profiles at a number of points along a transect in a restored landfill site in Essex correlated well with the measured emission of landfill gas from the surface of the soil . Emissions were only detected where methane concentrations reached the soil surface . Key Words-Landfill gas; sampling apparatus, concentration gradients in soils : methane oxidation, methanotrophic bacteria .
1.
Introduction of of this gas is methane
Landfill gas is produced by the microbial degradation of the organic components municipal refuse under anaerobic conditions . Typically 55-70%
while the balance is carbon dioxide . Trace amounts of other organic gases are also often present (Department
of Energy 1988) . Her Majesty's Inspectorate of Pollution (HMIP)
estimate that at least 60% of the 4000 landfill sites currently in operation and 75% of those closed within the last ten years produce methane, which has the potential to migrate out of the refuse body with adverse effects to areas remote from the landfill site (Milne 1988) . A number of recent events have brought the hazards associated with landfill-produced methane very much into public view . The best known of these was the destruction of a bungalow in Loscoe, Derbyshire in March 1986, when a spark from a central heating system ignited landfill gas which had migrated from a nearby landfill site to collect in the underfloor cavity of the building . To date, there has been only one reported fatality due to landfill gas in the U .K . but a great number of potentially lethal incidents have occurred . Circulars were issued to waste disposal authorities in England and Wales requiring that all of the landfill sites in Britain, whether completed or still operational, be surveyed in order to determine the exact scale of methane production (Her Majesty's Inspectorate of Pollution 1987) . Possibly the most widely used surveillance technique for landfill-produced methane is the regular on-site monitoring of the gas composition within permanent bore-holes sunk into the refuse body, using a methane detector . However, when gas samples are collected from bore holes there is no possibility of accurately determining their precise source, or in determining their variation with depth . Alternatively, if samples are taken using
* Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2BX. ** Department of Biology, University of Essex, Wivenhoe Park, Colchester, Essex C04 3SQ, U .K . 0734-242X/90/010021 + 11 303 .00/0
e1990ISWA
22
H. A . Jones & D . B . Nedwell
hollow gas probes they can be "pulled" from anywhere in the soil profile . The accurate monitoring of the changes in soil atmosphere with depth can only be carried out using equilibration chambers situated at the required point in the soil profile . Methane is produced in large quantities from a very wide range of sources beside landfill sites . The annual global release of methane into the atmosphere is estimated at between 5 .5 x 10 14 and 11 x 10 14 g CH4 each year . At least 80% of this is derived from biological activity (principally in swamps, marshes, paddy-fields and enteric fermentation in animals) while the remainder is anthropogenic in origin (principally from coal mining and industrial losses) (Ehhalt 1976) . A substantial, although unknown, proportion of the annual global production of biomass is decomposed under anaerobic conditions : it is estimated that through these processes 50%o of the dry weight of this unknown fraction of the world's organic matter is converted to methane . However, the annual emission of between 5 .5 x 10 14 and 11 x 10 14 g CH, every year into the atmosphere accounts for only 0 .5% (dry weight) of the annual global production of biomass . By far the majority of the annual methane production must be oxidized to carbon dioxide by aerobic bacteria in the environment before it can reach the atmosphere (Higgins et al . 1981) . Methane oxidation is carried out by a group of micro-organisms known as methylotrophs, defined as those organisms able to gain energy from the oxidation of reduced carbon compounds which may contain one or more carbon atoms, but no carboncarbon bonds (Colby & Zatman 1972) . Anthony (1982) distinguished methylotrophs from methanotrophs, which are those organisms possessing the specific enzyme which allows them to oxidize methane, the most reduced of the single-carbon compounds . Over 100 species of methylotrophic bacteria have been isolated . They are both numerous and widespread in nature (Whittenbury et al. 1970), and have been isolated from soils, sediments and fresh and marine water in a very wide range of environments . Most have an obligate requirement for reduced single-carbon substrates and are gramnegative, obligately aerobic rods, vibrio or cocci . Their activity depends upon the presence of sufficiently high concentrations of both methane and oxygen, and so they tend to be confined to fairly narrow horizontal bands within their habitat, limited in their distribution by the downward diffusion of atmospheric oxygen and the upward diffusion of biogenic methane . The precise determination of vertical concentration profiles of methane and oxygen is therefore important in examining the oxidation of methane in the field . Methane oxidation in aquatic environments is thought to account for a large proportion of the global oxidation of methane . Methane oxidizing bacteria in freshwater environments were shown to be responsible for oxidizing 60% of the methane production of a small freshwater lake, while the remaining 40% escaped to the atmosphere (Rudd & Hamilton 1975) . Although methane oxidizing bacteria are ubiquitous in terrestrial environments, their contribution to the terrestrial cycling of carbon is less clear (Hanson 1980) . Methane oxidizing activity, with a decrease in soil oxygen and an increase in microbial biomass, has been demonstrated in soils around leaks in natural gas pipes (Adamse et al. 1972 : Adams & Ellis 1960) . Methane oxidation has also been shown to occur in the soils above and adjacent to landfill sites (Mancinelli et al . 1981) . Mancinelli and McKay (1985) estimated that approximately 10% of the methane produced by a landfill is oxidized by the methylotrophic bacteria present in the top cover, but gave no indication of the limiting factors preventing complete oxidation of the methane . Measurements of the emission of methane through the restoration cover of a
Concentration profiles and emission rates above landfill sites
23
completed waste disposal site, and the distributions of methane and oxygen in the top 36 cm of the soil cover, have been made in an investigation into the behaviour of landfill gas in soils . A sampling programme on this site has been developed to study the extent to which the soil atmosphere is affected by the methanotrophs present in the soil . To this end, it has been necessary to develop a method of collecting small samples of soil atmosphere which reflect accurately the distribution of methane and oxygen in the surface soils . 2. Methods 2 .1 Study site
The site under study is a completed and restored area of a municipal landfill site in Essex (U.K.) (Martin's Farm Landfill Site, National Grid Reference TM 117177), the remainder of which is still in operation . The completed area of the site was partially clay capped, and was completed with a thin restoration cover (approximately 40-60 cm) of a sandy loam which was infertile and lacked structure, drainage and water holding capacity (Davis 1988) . A contour profile of a surveyed cross-section of the site is shown in Figure 1, which also shows the positions of four sampling sites which are being studied along this transect . These sites were selected on the basis of preliminary measurements of methane distributions, and covered the range of distribution patterns of methane and oxygen found on this landfill. None of these sampling sites are above clay-capped areas of the fill . 2.2 Concentration profiles of gases
The vertical concentration profiles of methane and oxygen in the soil atmosphere were sampled with an apparatus derived from that described by Hesslein (1976) and used aquatic sediments (Winfrey & Zeikus 1977) . Chambers (1 cm diameter x 1 cm deep) were drilled at 1 .5 cm intervals for 36 cm along the length of a stainless steel rod of approximately 2.5 cm diameter (Fig . 2) . Stainless steel was used in place of the original perspex (Hesslein 1976) in order to be sufficiently robust to withstand being driven into landfill cover. A stainless steel cover plate, with small equilibration holes over the machined cavities, was used to hold a gas permeable polyethylene membrane in place 81027 / Fence 20
0 0
â
N m 10
0
Site I Black silty subsoil 54 cm depth
Site 2 Silty clay with rubble 50 cm depth
LANDFILL ROAD Site 3 Brown stony topsoil 37 cm depth Site 4 Stiff brown/ blue clay 50 cm depth /DITCH
I I I I I 100 200 300 400 500 Metres
Fig . 1 . A surveyed transect of the study site, showing the positions of the sampling points . (A .O .D ., Above Ordinance Datum)
24
H. A . Jones & D . B . Nedwell 11 0 D Sharpened stainless steel rod
Drilled cavity
Silicone robber
Gas permeable membrane
Perforated steel facing plate
EI EI 0
Fig. 2 . Gas sampling stake .
over the machined cavities . This membrane was glued onto the face of the metal stake using silicone rubber, to prevent the ingress of soil water . The cover plate was firmly screwed down to protect the membrane, and the stake was then hammered into the ground at the required location . The stake was left in place to equilibrate with the soil gas atmosphere for 48 hr, after which it was recovered, and sealed with two layers of electrical insulating tape which prevented loss of gases, but allowed easy removal of samples with a gas-tight syringe (see below) . The concentrations of methane and oxygen in each of the cavities were analysed by gas chromatography . 2 .3 Permeability and gas exchange across membrane
In the original description of the apparatus (Hesslein 1976 ; Winfrey & Zeikus 1977) a cellulose dialysis membrane was used . Cellulose may be subjected to microbial attack and, particularly in a landfill environment, it was felt that a polyethylene membrane might be more suitable . A variety of inexpensive, widely available proprietary brands of polyethylene were tested for their permeability to the gases studied (methane, carbon dioxide, oxygen and nitrogen) by measuring the rate of diffusion of methane and carbon dioxide across each of the different membranes into air-filled glass vials . A number of these vials were removed after various time intervals, and the increase in methane or carbon dioxide concentrations within the vials with time was determined by gas chromatography . The sampling apparatus was tested under simulated sampling conditions by placing a
Concentration profiles and emission rates above landfill sites
25
stake in a container under a constant stream of methane for 24 hrs . The stake was then removed, and the equilibration holes in the facing plate were sealed with two thicknesses of electrical insulating tape . The sample retention was measured by analysing the gas composition within the chambers at half-hourly intervals after the removal of the stake . 2.4 Emission of methane from the surface soil Small polyethylene boxes (24 .5 X 21 .5 X 7 .5 cm) were inverted over the surface of the soil at each site and pressed into the soil surface . A hole drilled in the upper surface of each box was plugged with a Suba-seal (FSA, U .K .) through which gas samples could be withdrawn, using a hypodermic syringe and needle . Sub-samples (5 ml) of the gas atmosphere were taken from each box at 30 min intervals with polypropylene hypodermic syringes, which were immediately sealed . Each gas sample was subsequently analysed for methane concentration on return to the laboratory . 2 .5 Gas chromatography A gas chromatograph (Carlo Erba Fractovap model 4200) was used for the measurement of methane and oxygen in the gas samples collected on site . Methane concentrations were measured using a flame ionization detector . Samples (50 µl) were injected into a 3 m x 3 mm (i .d .) glass column packed with silica gel (80/100 mesh), with nitrogen as carrier gas at a flow rate of 15 ml min - ' at a column temperature of 100 ° C, and injection port and detector temperatures of 150°C . For gas samples collected from the methane traps, an injection loop was used to sub-sample 500 µl of the gas from the polypropylene syringes . The detection limit for methane under these conditions was 0 .01 .tmol ml -' . Oxygen was measured in the same gas chromatograph with a thermal conductivity detector, using a 3 m x 2 mm (i .d .) brass column packed with Carbonsieve S (supplied by Phase Separations Ltd ., U . K .) . Helium was used as carrier gas, at a flow rate of 25 ml min - ' . The column and injection port were maintained at a temperature of 50 ° C, and the thermal conductivity detector filament at 200 ° C . The detection limit for oxygen was 0.05 gmol ml - ' . 2 .6 Sampling strategy At each site triplicate gas sampling stakes, and triplicate surface methane traps were positioned randomly over a 10 X 10 m grid around a permanent marker at each sampling site . One gas sampling stake and one methane trap were sited at each of the randomly selected co-ordinates . Sampling started in April, 1988, and was repeated at monthly intervals . Soil temperature was also recorded at a depth of 10 cm at each site using a mercury thermometer. Soil moisture contents were determined gravimetrically by heating samples of surface soil (0-5 cm) to constant weight at 105°C .
3 . Results Figure 3 shows the increase in the concentrations of methane and carbon dioxide within glass vials sealed with a variety of polyethylene films . The most permeable of the films tested was Saran-wrap (Dow Chemical Co .) : within 2 hrs of equilibration, the air within the vials was completely replaced by methane or carbon dioxide (allowing for conversion to standard temperature and pressure, a concentration of approximately 44 .5 pmol ml - '
26
H. A . Jones & D . B . Nedwell Methane diffusion
50
É v 0 E
40
30
30
60
90
120
Time (min)
Carbon dioxide diffusion
Fig. 3 . The relative gas permeability of various polyethylene films showing diffusion of (a) Methane and (b) Carbon dioxide into air-filled vials . Key : O, Saran-Wrap (Dow Chemical Co .) ; •, Nesco film (Bando Chemical Ind . Ltd ., Japan); A, Black polythene (Refuse bags, FSA, U .K .) ; and A, Electrical insulating tape (Radio Spares) .
is equivalent to a pure gas) . The least permeable of the films tested was electrical insulating tape (Radio Ind . Ltd ., Japan) . On the basis of this experiment, Saran Wrap was selected as the equilibration membrane . Since complete equilibration of all of the gases measured (methane, carbon dioxide, oxygen and nitrogen) occurred through this membrane in under 2 hrs, it was decided that an equilibration period of 48 hrs would be more than sufficient to ensure that the atmosphere within the chamber was a true reflection of the soil atmosphere, and that the air introduced into the soil profile within the sample chamber would not continue to give elevated oxygen readings on analysis of the contents of the stake . Any variations in the thickness of the equilibration membrane, giving rise to differences in the rate of diffusion of gases through it, would also be compensated for by this extended equilibration period . Under simulated test conditions, the sampling chambers within the stake were found
Concentration profiles and emission rates above landfill sites
27
to have equilibrated to contain an atmosphere of 100% methane within 24 hrs equilibration (Fig. 4) . The use of two thicknesses of electrical insulating tape as a seal preserved the samples unchanged for at least 2 .5 hrs, with no detectable ingress of air into the chamber . Since according to the results presented in Fig . 3 the permeability of Saran-Wrap to methane and carbon dioxide is similar, it was not thought necessary to repeat this test with either carbon dioxide or a mixture of the two gases . Transport of the stakes back to the laboratory, and analyses of the gas samples collected in this way, were always completed within 2-3 hrs of recovery . Characteristic methane and oxygen profiles from each of the four sites under study are illustrated in Fig. 5 . Site 1 had methane at depths of 10-20 cm and below, but rarely at its surface. Sites 2 and 3 showed methane present throughout their profiles, but characteristically with slightly higher concentrations of methane at all depths in Site 3 than in Site 2 . Site 4 rarely showed detectable levels of methane at any of the depths sampled . The examples shown in Fig . 5 represent the average data for the three replicates at each of the four sites for September 1988 . Figure 6 show the seasonal changes in the measured rates of methane emission from the surface of the restoration cover, using three traps placed at the same randomly chosen co-ordinates as the gas sampling stakes . Figures 7 and 8 show the soil temperatures and soil moisture contents at these sites over the same period . The emission rates for methane at each of the sites were related to the measured concentration profiles for methane at the same sites . Where methane was not found at the surface of the soil profile, methane was never released from the soil surface . At sites where methane was detected at the soil surface, the emission rate tended to be proportional to the concentration of methane at the soil surface . Sites 2 and 3 have both shown increases in the methane emission rates over the warmest and driest months of the summer . The maximum rate of methane emission to date has been measured at Site 3 in September 1988 as 585 mmoles CH4 m-2 hr - ' (at S . T. P ., approximately 1 I litres m -2 hr - ') with the lowest emission rates measured in December as 0 .25 mmoles CH4 m -2 hr - ' at Site 2 and 3 .43 mmoles CH4 m -2 hr - ' at Site 3 . Methane emission from the surface of Sites 1 and 4 has never been detected during the course of this study .
50
40
e-e-e\ e
A,
e
o 10
J
000 0
2
4
6
8
10
12
Time (hr) Fig . 4 . Sample storage in gas sampling stake . Key : Z~, Methane; O, Oxygen ; and •,
Nitrogen .
H . A . Jones & D . B . Nedwell
28
Site 2 0
10
2
0 --
Concentration (µmoles ml 4 6 r
-
) 8
0 1 -.
10, o
-6 -12 E o L â d 0
-18 -24 0 -30
o
-36 -
Site 3
Site 4
Concentration (µmoles ml -1 ) 10 15 5
0
Concentration (µmoles ml 2 4 6
-1 ) 8
10
0
s
-6
'a
w -1
12
E o
E o
0
L -18a, â 0 O -24o o
L -18 â -24 -30
-30o
-36
-36o
o
Fig . 5 . Methane and oxygen concentration profiles in the top 36 cm of the restoration cover of a methaneproducing landfill site. Key : O, Methane ; and •, Oxygen .
I
700 L
600 E 500 = U
N
û 400 o E E 300 v °3 200
o
o
100
dl
wE
o Jul
Fig. 6 . Surface methane emission rates . Key:
Aug
•, Site
Sep
Oct
Nov
Dec
1 ; A, Site 2; ∎, Site 3 ; and
•, Site 4 .
Concentration profiles and emission rates above landfill sites
29
30
µ
a
Jul
a
a a
Sep
Oct
â
Aug
Fig . 7. Soil temperatures (10 cm depth) . Key :
Nov
Dec
â, Site 1 ; A, Site 2 ; µ, Site 3; and +, Site 4 .
40
30 U) m 0
a,
20
v 3
10
â i
I
Jul
Aug
Sep
Oct
Fig. 8 . Soil moisture contents (% weight loss on drying at 105'C) . Key : +, Site 4.
Nov
Dec
â, Site 1 ; A, Site 2; µ, site 3 ; and
4. Discussion Methane oxidation by methanotrophic micro-organisms can only take place in env ments where methane and oxygen occur simultaneously . The measured gas concentration profiles indicated that at Site 1, although methane was present in the soil at depths of 15 cm and below, it was present in levels too low for detection (< 0 .01 tmol ml ') nearer the surface, and there was no emission from the soil surface . The soil cover at this site is a black silty subsoil of approximately 55 cm depth, and there is no evidence that differences in the permeability of this cover are responsible for the distribution of the landfill gases at this site . This strongly suggests that at this site any vertically migrating methane was oxidized in the upper layers of the soil before it reached the surface . Further work is being carried out to investigate this .
30
H . A . Jones & D . B. Nedwell
At Sites 2 and 3 methane was present in fairly high concentrations throughout the cover and migrated upwards, with a proportion escaping from the surface . In contrast, at site 4, methane was present only in very low concentrations, possibly as a result of the high clay content of the soil at this site . The oxygen profiles of the four sites may also suggest that biological activity is responsible for some of the disappearance of methane at Site 1 . At Sites I and 4, the concentration of oxygen 1 .5 cm from the surface is approximately equal to that of atmospheric oxygen, but the decrease in the concentration of oxygen with depth at Site I is more rapid than at Site 4, implying that aerobic microbial activity may be responsible for the decrease in the concentration of both the methane as it travels upwards and the oxygen as it diffuses downwards . It is, however, also possible that differences in the permeability of the soil are responsible for the differences in oxygen penetration . Direct measurements of methane oxidation will resolve this question . At Sites 2 and 3, the poor oxygen penetration is likely to be due to the displacement of the normal soil atmosphere by the upward movement of the landfill gas. On the evidence of these data, there is no reason to suppose that the disappearance of methane as it nears the surface of the soil cover is due to any biological activity . It is entirely possible that the effect seen is simply one of dilution with air . The levels of temperature and more particularly moisture at these sites may also reveal some of the reasons for the different behaviour of the methane at these sites . Over the soil temperature never fell below 7âC, and over the summer months temperatures of around 14âC were recorded at Sites 1, 2 and 4 . At Site 3, however, extremely elevated temperatures of up to 23âC were recorded . It is not clear whether these high surface temperatures were due to elevated temperatures within the refuse body, or were a product of microbial activity within the cover soil . Soil moisture contents showed a typical seasonal pattern, with the soils at their driest in August and their wettest in November and December . The peak of methane emission at Sites 2 and 3 occurred from July to September, coinciding with the warmest and driest soil conditions . Assuming that the increase in methane emission over the summer was not due to increased methane production from the refuse body (methane production at depth in landfills is generally thought to be independent of external seasonal temperature changes : an increase in methane production in winter, with increased rainwater infiltration into the refuse, can, however, be expected (Farquhar & Rovers 1973)) . It is possible that the lack of soil moisture in the four sites was responsible for a decrease in the activity of the methanotrophic soil microorganisms, which in turn led to an increase in the rate of emission of methane from the soil surface . The conventional methods of monitoring landfill methane concentrations give no information about the potential fate of methane under natural conditions . The techniques applied here have shown significant differences between the four sites within this landfill site permit the precise determination of detailed gas concentration profiles in soils above or adjacent to landfill sites, and facilitate the location of sites where methane emission or methane oxidation may be occurring . Further work is now under way to develop methods to measure directly methane oxidation rates in these soils and to count the number of methane-oxidizing bacteria present . Preliminary studies of the population of methane-oxidizing micro-organisms in the surface soil of these sites have shown that the numbers of methanotrophic bacteria correlate with the average methane concentrations at the surface of the cover, with Site 3 harbouring the highest number of methanotrophs per gram of soil .
Concentration profiles and emission rates above landfill sites
31
5 . Acknowledgments This work was part of a project funded by the U . K . Department of the Environment, contract No . PECD 7/10/96 . We thank Essex County Council for permission to use the sampling site, and for the data reproduced in Fig . 1 . References Adams R . S . & Ellis, R . (1960), Some physical and chemical changes in soils brought about by natural gas . In Soil Science Society Proceedings, 1960, 41-44 . Adamse, A. D., Hoeks, J ., DeBont, 3 . A. M. & Van Kessel, J . F . (1972), Microbial a t near natural gas leaks . Archiv fur Mikrobiologie, 83, 31-35 . Anthony, C . (1982), The Biochemistry of Methylotrophs . Academic Press, London . Colby, J . & Zatman, L . J . (1972), Hexose phosphate synthase and tricarboxylic acid enzymes in bacterium 4B6, an obligate methylotroph . Biochemical Journal, 128, 1373-1376. Davis, B. N . K . (1988), Habitat creation on a landfill site . Mine and Quarry Environment, 2,29-32 . Department of Energy (1988), A Basic Study of Landfill Microbiology and Biochemistry . Report ETSU B 1159 . Ehhalt, D . H . (1976), The atmospheric cycle of methane . In Symposium on Miocrobial Production and Utilisation of Gases (HZ , CH, CO), (H . G . Schlegel, G. Gottschalk, & N . Pfennig, eds) . Akademie der Wissenschaften, Gottingen . Farquhar, G . J . & Rovers, F. A . (1973), Gas production during refuse decomposition . Water, Air and Soil Pollution, 2, 483-495 . Hanson, R . S . (1980), Ecology and diversity of methylotrophic microorganisms . Journal of Applied Bacteriology, 26, 3-39 . Her Majesty's Inspectorate of Pollution, (1987), News Release Safe Management of Landfill Sites . 544 .
Hesslein, R . H . (1976), An in situ sampler for close interval pore water studies . Limnology and Oceanography, 21, 912-914 . Higgins, 1 . J ., Best, D. J ., Hammond, R. C. & Scott, D . (1981), Methane oxidising microorganisms . Microbiological Reviews, 45, 556-590 . Mancinelli, R. L. & McKay, C . P . (1985), Methane oxidising bacteria in sanitary landfills . In Biotechnological Advances in Processing Municipal Wastes for Fuels and Chemicals, (A . A . Antonopoulos, ed .) Argonne National laboratory, Argonne, Illinois. Mancinelli, R. L ., Shulls, W . A . & McKay, C . P . (1981), Methane oxidising bacteria used as an index of soil methane concentration . Applied and Environmental Microbiology, 42, 70--73 . Milne, R . (1988), Methane menace seeps to the surface . New Scientist, 117 (issue 1601), 27 . Rudd, J . W . & Hamilton, R. D . (1975), Factors controlling rates of methane oxidation and the distribution of methane oxidisers in a small stratified lake . Archives of Hydrobiology, 75, 552-538 . Whittenbury, R ., Phillips, K . C . & Wilkinson, J . F . (1970), Enrichment, isolation and some properties of methane-utilising bacteria . Journal of General Microbiology, 61, 205-218. Winfrey, M. R . & Zeikus, J . G. (1977), Effect of sulfate on carbon and electron flow during microbial methanogenesis in freshwater sediments . Applied and Environmental Microbiology, 33, 275-281 .
SOIL ATMOSPHERE CONCENTRATION PROFILES AND METHANE EMISSION RATES IN THE RESTORATION COVERS ABOVE LANDFILL SITES : EQUIPMENT AND PRELIMINARY RESULTS Hilary A . Jones* and D. B. Nedwell** (Received March 1989, revised July 1989) In an investigation into the microbial oxidation of methane in soils, a simple and robust sampling device for the collection of small samples of soil atmosphere was used. The method has the potential for determining small-scale variations in depth profiles of gases, without the potential problems of sample migration during collection . Measurements of methane concentration profiles at a number of points along a transect in a restored landfill site in Essex correlated well with the measured emission of landfill gas from the surface of the soil . Emissions were only detected where methane concentrations reached the soil surface . Key Words-Landfill gas; sampling apparatus, concentration gradients in soils : methane oxidation, methanotrophic bacteria .
1.
Introduction of of this gas is methane
Landfill gas is produced by the microbial degradation of the organic components municipal refuse under anaerobic conditions . Typically 55-70%
while the balance is carbon dioxide . Trace amounts of other organic gases are also often present (Department
of Energy 1988) . Her Majesty's Inspectorate of Pollution (HMIP)
estimate that at least 60% of the 4000 landfill sites currently in operation and 75% of those closed within the last ten years produce methane, which has the potential to migrate out of the refuse body with adverse effects to areas remote from the landfill site (Milne 1988) . A number of recent events have brought the hazards associated with landfill-produced methane very much into public view . The best known of these was the destruction of a bungalow in Loscoe, Derbyshire in March 1986, when a spark from a central heating system ignited landfill gas which had migrated from a nearby landfill site to collect in the underfloor cavity of the building . To date, there has been only one reported fatality due to landfill gas in the U .K . but a great number of potentially lethal incidents have occurred . Circulars were issued to waste disposal authorities in England and Wales requiring that all of the landfill sites in Britain, whether completed or still operational, be surveyed in order to determine the exact scale of methane production (Her Majesty's Inspectorate of Pollution 1987) . Possibly the most widely used surveillance technique for landfill-produced methane is the regular on-site monitoring of the gas composition within permanent bore-holes sunk into the refuse body, using a methane detector . However, when gas samples are collected from bore holes there is no possibility of accurately determining their precise source, or in determining their variation with depth . Alternatively, if samples are taken using
* Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2BX. ** Department of Biology, University of Essex, Wivenhoe Park, Colchester, Essex C04 3SQ, U .K . 0734-242X/90/010021 + 11 303 .00/0
e1990ISWA
22
H. A . Jones & D . B . Nedwell
hollow gas probes they can be "pulled" from anywhere in the soil profile . The accurate monitoring of the changes in soil atmosphere with depth can only be carried out using equilibration chambers situated at the required point in the soil profile . Methane is produced in large quantities from a very wide range of sources beside landfill sites . The annual global release of methane into the atmosphere is estimated at between 5 .5 x 10 14 and 11 x 10 14 g CH4 each year . At least 80% of this is derived from biological activity (principally in swamps, marshes, paddy-fields and enteric fermentation in animals) while the remainder is anthropogenic in origin (principally from coal mining and industrial losses) (Ehhalt 1976) . A substantial, although unknown, proportion of the annual global production of biomass is decomposed under anaerobic conditions : it is estimated that through these processes 50%o of the dry weight of this unknown fraction of the world's organic matter is converted to methane . However, the annual emission of between 5 .5 x 10 14 and 11 x 10 14 g CH, every year into the atmosphere accounts for only 0 .5% (dry weight) of the annual global production of biomass . By far the majority of the annual methane production must be oxidized to carbon dioxide by aerobic bacteria in the environment before it can reach the atmosphere (Higgins et al . 1981) . Methane oxidation is carried out by a group of micro-organisms known as methylotrophs, defined as those organisms able to gain energy from the oxidation of reduced carbon compounds which may contain one or more carbon atoms, but no carboncarbon bonds (Colby & Zatman 1972) . Anthony (1982) distinguished methylotrophs from methanotrophs, which are those organisms possessing the specific enzyme which allows them to oxidize methane, the most reduced of the single-carbon compounds . Over 100 species of methylotrophic bacteria have been isolated . They are both numerous and widespread in nature (Whittenbury et al. 1970), and have been isolated from soils, sediments and fresh and marine water in a very wide range of environments . Most have an obligate requirement for reduced single-carbon substrates and are gramnegative, obligately aerobic rods, vibrio or cocci . Their activity depends upon the presence of sufficiently high concentrations of both methane and oxygen, and so they tend to be confined to fairly narrow horizontal bands within their habitat, limited in their distribution by the downward diffusion of atmospheric oxygen and the upward diffusion of biogenic methane . The precise determination of vertical concentration profiles of methane and oxygen is therefore important in examining the oxidation of methane in the field . Methane oxidation in aquatic environments is thought to account for a large proportion of the global oxidation of methane . Methane oxidizing bacteria in freshwater environments were shown to be responsible for oxidizing 60% of the methane production of a small freshwater lake, while the remaining 40% escaped to the atmosphere (Rudd & Hamilton 1975) . Although methane oxidizing bacteria are ubiquitous in terrestrial environments, their contribution to the terrestrial cycling of carbon is less clear (Hanson 1980) . Methane oxidizing activity, with a decrease in soil oxygen and an increase in microbial biomass, has been demonstrated in soils around leaks in natural gas pipes (Adamse et al. 1972 : Adams & Ellis 1960) . Methane oxidation has also been shown to occur in the soils above and adjacent to landfill sites (Mancinelli et al . 1981) . Mancinelli and McKay (1985) estimated that approximately 10% of the methane produced by a landfill is oxidized by the methylotrophic bacteria present in the top cover, but gave no indication of the limiting factors preventing complete oxidation of the methane . Measurements of the emission of methane through the restoration cover of a
Concentration profiles and emission rates above landfill sites
23
completed waste disposal site, and the distributions of methane and oxygen in the top 36 cm of the soil cover, have been made in an investigation into the behaviour of landfill gas in soils . A sampling programme on this site has been developed to study the extent to which the soil atmosphere is affected by the methanotrophs present in the soil . To this end, it has been necessary to develop a method of collecting small samples of soil atmosphere which reflect accurately the distribution of methane and oxygen in the surface soils . 2. Methods 2 .1 Study site
The site under study is a completed and restored area of a municipal landfill site in Essex (U.K.) (Martin's Farm Landfill Site, National Grid Reference TM 117177), the remainder of which is still in operation . The completed area of the site was partially clay capped, and was completed with a thin restoration cover (approximately 40-60 cm) of a sandy loam which was infertile and lacked structure, drainage and water holding capacity (Davis 1988) . A contour profile of a surveyed cross-section of the site is shown in Figure 1, which also shows the positions of four sampling sites which are being studied along this transect . These sites were selected on the basis of preliminary measurements of methane distributions, and covered the range of distribution patterns of methane and oxygen found on this landfill. None of these sampling sites are above clay-capped areas of the fill . 2.2 Concentration profiles of gases
The vertical concentration profiles of methane and oxygen in the soil atmosphere were sampled with an apparatus derived from that described by Hesslein (1976) and used aquatic sediments (Winfrey & Zeikus 1977) . Chambers (1 cm diameter x 1 cm deep) were drilled at 1 .5 cm intervals for 36 cm along the length of a stainless steel rod of approximately 2.5 cm diameter (Fig . 2) . Stainless steel was used in place of the original perspex (Hesslein 1976) in order to be sufficiently robust to withstand being driven into landfill cover. A stainless steel cover plate, with small equilibration holes over the machined cavities, was used to hold a gas permeable polyethylene membrane in place 81027 / Fence 20
0 0
â
N m 10
0
Site I Black silty subsoil 54 cm depth
Site 2 Silty clay with rubble 50 cm depth
LANDFILL ROAD Site 3 Brown stony topsoil 37 cm depth Site 4 Stiff brown/ blue clay 50 cm depth /DITCH
I I I I I 100 200 300 400 500 Metres
Fig . 1 . A surveyed transect of the study site, showing the positions of the sampling points . (A .O .D ., Above Ordinance Datum)
24
H. A . Jones & D . B . Nedwell 11 0 D Sharpened stainless steel rod
Drilled cavity
Silicone robber
Gas permeable membrane
Perforated steel facing plate
EI EI 0
Fig. 2 . Gas sampling stake .
over the machined cavities . This membrane was glued onto the face of the metal stake using silicone rubber, to prevent the ingress of soil water . The cover plate was firmly screwed down to protect the membrane, and the stake was then hammered into the ground at the required location . The stake was left in place to equilibrate with the soil gas atmosphere for 48 hr, after which it was recovered, and sealed with two layers of electrical insulating tape which prevented loss of gases, but allowed easy removal of samples with a gas-tight syringe (see below) . The concentrations of methane and oxygen in each of the cavities were analysed by gas chromatography . 2 .3 Permeability and gas exchange across membrane
In the original description of the apparatus (Hesslein 1976 ; Winfrey & Zeikus 1977) a cellulose dialysis membrane was used . Cellulose may be subjected to microbial attack and, particularly in a landfill environment, it was felt that a polyethylene membrane might be more suitable . A variety of inexpensive, widely available proprietary brands of polyethylene were tested for their permeability to the gases studied (methane, carbon dioxide, oxygen and nitrogen) by measuring the rate of diffusion of methane and carbon dioxide across each of the different membranes into air-filled glass vials . A number of these vials were removed after various time intervals, and the increase in methane or carbon dioxide concentrations within the vials with time was determined by gas chromatography . The sampling apparatus was tested under simulated sampling conditions by placing a
Concentration profiles and emission rates above landfill sites
25
stake in a container under a constant stream of methane for 24 hrs . The stake was then removed, and the equilibration holes in the facing plate were sealed with two thicknesses of electrical insulating tape . The sample retention was measured by analysing the gas composition within the chambers at half-hourly intervals after the removal of the stake . 2.4 Emission of methane from the surface soil Small polyethylene boxes (24 .5 X 21 .5 X 7 .5 cm) were inverted over the surface of the soil at each site and pressed into the soil surface . A hole drilled in the upper surface of each box was plugged with a Suba-seal (FSA, U .K .) through which gas samples could be withdrawn, using a hypodermic syringe and needle . Sub-samples (5 ml) of the gas atmosphere were taken from each box at 30 min intervals with polypropylene hypodermic syringes, which were immediately sealed . Each gas sample was subsequently analysed for methane concentration on return to the laboratory . 2 .5 Gas chromatography A gas chromatograph (Carlo Erba Fractovap model 4200) was used for the measurement of methane and oxygen in the gas samples collected on site . Methane concentrations were measured using a flame ionization detector . Samples (50 µl) were injected into a 3 m x 3 mm (i .d .) glass column packed with silica gel (80/100 mesh), with nitrogen as carrier gas at a flow rate of 15 ml min - ' at a column temperature of 100 ° C, and injection port and detector temperatures of 150°C . For gas samples collected from the methane traps, an injection loop was used to sub-sample 500 µl of the gas from the polypropylene syringes . The detection limit for methane under these conditions was 0 .01 .tmol ml -' . Oxygen was measured in the same gas chromatograph with a thermal conductivity detector, using a 3 m x 2 mm (i .d .) brass column packed with Carbonsieve S (supplied by Phase Separations Ltd ., U . K .) . Helium was used as carrier gas, at a flow rate of 25 ml min - ' . The column and injection port were maintained at a temperature of 50 ° C, and the thermal conductivity detector filament at 200 ° C . The detection limit for oxygen was 0.05 gmol ml - ' . 2 .6 Sampling strategy At each site triplicate gas sampling stakes, and triplicate surface methane traps were positioned randomly over a 10 X 10 m grid around a permanent marker at each sampling site . One gas sampling stake and one methane trap were sited at each of the randomly selected co-ordinates . Sampling started in April, 1988, and was repeated at monthly intervals . Soil temperature was also recorded at a depth of 10 cm at each site using a mercury thermometer. Soil moisture contents were determined gravimetrically by heating samples of surface soil (0-5 cm) to constant weight at 105°C .
3 . Results Figure 3 shows the increase in the concentrations of methane and carbon dioxide within glass vials sealed with a variety of polyethylene films . The most permeable of the films tested was Saran-wrap (Dow Chemical Co .) : within 2 hrs of equilibration, the air within the vials was completely replaced by methane or carbon dioxide (allowing for conversion to standard temperature and pressure, a concentration of approximately 44 .5 pmol ml - '
26
H. A . Jones & D . B . Nedwell Methane diffusion
50
É v 0 E
40
30
30
60
90
120
Time (min)
Carbon dioxide diffusion
Fig. 3 . The relative gas permeability of various polyethylene films showing diffusion of (a) Methane and (b) Carbon dioxide into air-filled vials . Key : O, Saran-Wrap (Dow Chemical Co .) ; •, Nesco film (Bando Chemical Ind . Ltd ., Japan); A, Black polythene (Refuse bags, FSA, U .K .) ; and A, Electrical insulating tape (Radio Spares) .
is equivalent to a pure gas) . The least permeable of the films tested was electrical insulating tape (Radio Ind . Ltd ., Japan) . On the basis of this experiment, Saran Wrap was selected as the equilibration membrane . Since complete equilibration of all of the gases measured (methane, carbon dioxide, oxygen and nitrogen) occurred through this membrane in under 2 hrs, it was decided that an equilibration period of 48 hrs would be more than sufficient to ensure that the atmosphere within the chamber was a true reflection of the soil atmosphere, and that the air introduced into the soil profile within the sample chamber would not continue to give elevated oxygen readings on analysis of the contents of the stake . Any variations in the thickness of the equilibration membrane, giving rise to differences in the rate of diffusion of gases through it, would also be compensated for by this extended equilibration period . Under simulated test conditions, the sampling chambers within the stake were found
Concentration profiles and emission rates above landfill sites
27
to have equilibrated to contain an atmosphere of 100% methane within 24 hrs equilibration (Fig. 4) . The use of two thicknesses of electrical insulating tape as a seal preserved the samples unchanged for at least 2 .5 hrs, with no detectable ingress of air into the chamber . Since according to the results presented in Fig . 3 the permeability of Saran-Wrap to methane and carbon dioxide is similar, it was not thought necessary to repeat this test with either carbon dioxide or a mixture of the two gases . Transport of the stakes back to the laboratory, and analyses of the gas samples collected in this way, were always completed within 2-3 hrs of recovery . Characteristic methane and oxygen profiles from each of the four sites under study are illustrated in Fig. 5 . Site 1 had methane at depths of 10-20 cm and below, but rarely at its surface. Sites 2 and 3 showed methane present throughout their profiles, but characteristically with slightly higher concentrations of methane at all depths in Site 3 than in Site 2 . Site 4 rarely showed detectable levels of methane at any of the depths sampled . The examples shown in Fig . 5 represent the average data for the three replicates at each of the four sites for September 1988 . Figure 6 show the seasonal changes in the measured rates of methane emission from the surface of the restoration cover, using three traps placed at the same randomly chosen co-ordinates as the gas sampling stakes . Figures 7 and 8 show the soil temperatures and soil moisture contents at these sites over the same period . The emission rates for methane at each of the sites were related to the measured concentration profiles for methane at the same sites . Where methane was not found at the surface of the soil profile, methane was never released from the soil surface . At sites where methane was detected at the soil surface, the emission rate tended to be proportional to the concentration of methane at the soil surface . Sites 2 and 3 have both shown increases in the methane emission rates over the warmest and driest months of the summer . The maximum rate of methane emission to date has been measured at Site 3 in September 1988 as 585 mmoles CH4 m-2 hr - ' (at S . T. P ., approximately 1 I litres m -2 hr - ') with the lowest emission rates measured in December as 0 .25 mmoles CH4 m -2 hr - ' at Site 2 and 3 .43 mmoles CH4 m -2 hr - ' at Site 3 . Methane emission from the surface of Sites 1 and 4 has never been detected during the course of this study .
50
40
e-e-e\ e
A,
e
o 10
J
000 0
2
4
6
8
10
12
Time (hr) Fig . 4 . Sample storage in gas sampling stake . Key : Z~, Methane; O, Oxygen ; and •,
Nitrogen .
H . A . Jones & D . B . Nedwell
28
Site 2 0
10
2
0 --
Concentration (µmoles ml 4 6 r
-
) 8
0 1 -.
10, o
-6 -12 E o L â d 0
-18 -24 0 -30
o
-36 -
Site 3
Site 4
Concentration (µmoles ml -1 ) 10 15 5
0
Concentration (µmoles ml 2 4 6
-1 ) 8
10
0
s
-6
'a
w -1
12
E o
E o
0
L -18a, â 0 O -24o o
L -18 â -24 -30
-30o
-36
-36o
o
Fig . 5 . Methane and oxygen concentration profiles in the top 36 cm of the restoration cover of a methaneproducing landfill site. Key : O, Methane ; and •, Oxygen .
I
700 L
600 E 500 = U
N
û 400 o E E 300 v °3 200
o
o
100
dl
wE
o Jul
Fig. 6 . Surface methane emission rates . Key:
Aug
•, Site
Sep
Oct
Nov
Dec
1 ; A, Site 2; ∎, Site 3 ; and
•, Site 4 .
Concentration profiles and emission rates above landfill sites
29
30
µ
a
Jul
a
a a
Sep
Oct
â
Aug
Fig . 7. Soil temperatures (10 cm depth) . Key :
Nov
Dec
â, Site 1 ; A, Site 2 ; µ, Site 3; and +, Site 4 .
40
30 U) m 0
a,
20
v 3
10
â i
I
Jul
Aug
Sep
Oct
Fig. 8 . Soil moisture contents (% weight loss on drying at 105'C) . Key : +, Site 4.
Nov
Dec
â, Site 1 ; A, Site 2; µ, site 3 ; and
4. Discussion Methane oxidation by methanotrophic micro-organisms can only take place in env ments where methane and oxygen occur simultaneously . The measured gas concentration profiles indicated that at Site 1, although methane was present in the soil at depths of 15 cm and below, it was present in levels too low for detection (< 0 .01 tmol ml ') nearer the surface, and there was no emission from the soil surface . The soil cover at this site is a black silty subsoil of approximately 55 cm depth, and there is no evidence that differences in the permeability of this cover are responsible for the distribution of the landfill gases at this site . This strongly suggests that at this site any vertically migrating methane was oxidized in the upper layers of the soil before it reached the surface . Further work is being carried out to investigate this .
30
H . A . Jones & D . B. Nedwell
At Sites 2 and 3 methane was present in fairly high concentrations throughout the cover and migrated upwards, with a proportion escaping from the surface . In contrast, at site 4, methane was present only in very low concentrations, possibly as a result of the high clay content of the soil at this site . The oxygen profiles of the four sites may also suggest that biological activity is responsible for some of the disappearance of methane at Site 1 . At Sites I and 4, the concentration of oxygen 1 .5 cm from the surface is approximately equal to that of atmospheric oxygen, but the decrease in the concentration of oxygen with depth at Site I is more rapid than at Site 4, implying that aerobic microbial activity may be responsible for the decrease in the concentration of both the methane as it travels upwards and the oxygen as it diffuses downwards . It is, however, also possible that differences in the permeability of the soil are responsible for the differences in oxygen penetration . Direct measurements of methane oxidation will resolve this question . At Sites 2 and 3, the poor oxygen penetration is likely to be due to the displacement of the normal soil atmosphere by the upward movement of the landfill gas. On the evidence of these data, there is no reason to suppose that the disappearance of methane as it nears the surface of the soil cover is due to any biological activity . It is entirely possible that the effect seen is simply one of dilution with air . The levels of temperature and more particularly moisture at these sites may also reveal some of the reasons for the different behaviour of the methane at these sites . Over the soil temperature never fell below 7âC, and over the summer months temperatures of around 14âC were recorded at Sites 1, 2 and 4 . At Site 3, however, extremely elevated temperatures of up to 23âC were recorded . It is not clear whether these high surface temperatures were due to elevated temperatures within the refuse body, or were a product of microbial activity within the cover soil . Soil moisture contents showed a typical seasonal pattern, with the soils at their driest in August and their wettest in November and December . The peak of methane emission at Sites 2 and 3 occurred from July to September, coinciding with the warmest and driest soil conditions . Assuming that the increase in methane emission over the summer was not due to increased methane production from the refuse body (methane production at depth in landfills is generally thought to be independent of external seasonal temperature changes : an increase in methane production in winter, with increased rainwater infiltration into the refuse, can, however, be expected (Farquhar & Rovers 1973)) . It is possible that the lack of soil moisture in the four sites was responsible for a decrease in the activity of the methanotrophic soil microorganisms, which in turn led to an increase in the rate of emission of methane from the soil surface . The conventional methods of monitoring landfill methane concentrations give no information about the potential fate of methane under natural conditions . The techniques applied here have shown significant differences between the four sites within this landfill site permit the precise determination of detailed gas concentration profiles in soils above or adjacent to landfill sites, and facilitate the location of sites where methane emission or methane oxidation may be occurring . Further work is now under way to develop methods to measure directly methane oxidation rates in these soils and to count the number of methane-oxidizing bacteria present . Preliminary studies of the population of methane-oxidizing micro-organisms in the surface soil of these sites have shown that the numbers of methanotrophic bacteria correlate with the average methane concentrations at the surface of the cover, with Site 3 harbouring the highest number of methanotrophs per gram of soil .
Concentration profiles and emission rates above landfill sites
31
5 . Acknowledgments This work was part of a project funded by the U . K . Department of the Environment, contract No . PECD 7/10/96 . We thank Essex County Council for permission to use the sampling site, and for the data reproduced in Fig . 1 . References Adams R . S . & Ellis, R . (1960), Some physical and chemical changes in soils brought about by natural gas . In Soil Science Society Proceedings, 1960, 41-44 . Adamse, A. D., Hoeks, J ., DeBont, 3 . A. M. & Van Kessel, J . F . (1972), Microbial a t near natural gas leaks . Archiv fur Mikrobiologie, 83, 31-35 . Anthony, C . (1982), The Biochemistry of Methylotrophs . Academic Press, London . Colby, J . & Zatman, L . J . (1972), Hexose phosphate synthase and tricarboxylic acid enzymes in bacterium 4B6, an obligate methylotroph . Biochemical Journal, 128, 1373-1376. Davis, B. N . K . (1988), Habitat creation on a landfill site . Mine and Quarry Environment, 2,29-32 . Department of Energy (1988), A Basic Study of Landfill Microbiology and Biochemistry . Report ETSU B 1159 . Ehhalt, D . H . (1976), The atmospheric cycle of methane . In Symposium on Miocrobial Production and Utilisation of Gases (HZ , CH, CO), (H . G . Schlegel, G. Gottschalk, & N . Pfennig, eds) . Akademie der Wissenschaften, Gottingen . Farquhar, G . J . & Rovers, F. A . (1973), Gas production during refuse decomposition . Water, Air and Soil Pollution, 2, 483-495 . Hanson, R . S . (1980), Ecology and diversity of methylotrophic microorganisms . Journal of Applied Bacteriology, 26, 3-39 . Her Majesty's Inspectorate of Pollution, (1987), News Release Safe Management of Landfill Sites . 544 .
Hesslein, R . H . (1976), An in situ sampler for close interval pore water studies . Limnology and Oceanography, 21, 912-914 . Higgins, 1 . J ., Best, D. J ., Hammond, R. C. & Scott, D . (1981), Methane oxidising microorganisms . Microbiological Reviews, 45, 556-590 . Mancinelli, R. L. & McKay, C . P . (1985), Methane oxidising bacteria in sanitary landfills . In Biotechnological Advances in Processing Municipal Wastes for Fuels and Chemicals, (A . A . Antonopoulos, ed .) Argonne National laboratory, Argonne, Illinois. Mancinelli, R. L ., Shulls, W . A . & McKay, C . P . (1981), Methane oxidising bacteria used as an index of soil methane concentration . Applied and Environmental Microbiology, 42, 70--73 . Milne, R . (1988), Methane menace seeps to the surface . New Scientist, 117 (issue 1601), 27 . Rudd, J . W . & Hamilton, R. D . (1975), Factors controlling rates of methane oxidation and the distribution of methane oxidisers in a small stratified lake . Archives of Hydrobiology, 75, 552-538 . Whittenbury, R ., Phillips, K . C . & Wilkinson, J . F . (1970), Enrichment, isolation and some properties of methane-utilising bacteria . Journal of General Microbiology, 61, 205-218. Winfrey, M. R . & Zeikus, J . G. (1977), Effect of sulfate on carbon and electron flow during microbial methanogenesis in freshwater sediments . Applied and Environmental Microbiology, 33, 275-281 .